Consolidate multiple 'none' values in box-shadow Sass mixin

[julien-deramond]

Description

This PR fixes an issue with the box-shadow mixin where multiple none values could result in invalid CSS combinations like box-shadow: none none none. The fix consolidates multiple none values (and other single-value keywords like initial, inherit, unset) into a single value.

The changes are:

• Modified box-shadow mixin to detect and consolidate single-value keywords
• Added comprehensive test suite to verify the fix and other edge cases

They prevent invalid CSS output while maintaining backward compatibility with existing usage of the mixin.

These changes are covered by new tests that verify:

• Single and multiple none values
• Other CSS keywords (initial, inherit, unset)
• Regular box-shadow values
• Mixed keyword and shadow values
• Null value handling
• $enable-shadows variable behavior

Hopefully, these tests cover pretty much every use cases that could happen, reasonably 🤞 Don't hesitate if you're thinking about any other weird combinations; we could add them to the test suite.

I've also tested manually to compile this branch with $enable-shadows: true and compare the generated bootstrap.css with the one generated from the main branch with $enable-shadows: true. Same output.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• (N/A) My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Related issues

Closes #41381